I've had the same arrangement that JJM has with five different owners/horses over the past seven or eight years. Every one of them has been a great learning experience for me, and, I hope, worked out well for the horse and owner (I'm still in contact with several of them). As soon as the owner was able to ride their horse again, whether it was recovering from an injury or busy period at work, whatever, then they took the horse back. If you're comfortable with it, maybe something like that could be a good solution for you. Obviously, though, check out anyone you get pretty carefully.